I recently did this and got .25% reduction on the margin on my tracker between what my original offer was and the final one.
I actually hadn't planned to be a tart, but a Dutch auction developed where two institutions who had given me quotes keep knocking their price. At one stage, one bank told me they couldn't match the rate of the other so I thanked them and resolved mentally to go with the other. 10 mins later they rang back with another sliver knocked off. I eventually went with them.
It was all pretty stressful, but will save me a lot of money. Having been on the customer side of a few auctions, it was good to be the beneficiary for once.
Not sure everyone could do this, my circumstances were quite particular.